1. Field of the Invention
The present invention relates to an exposure light source device in reproducing apparatus or the like.
2. Description of the Prior Art
Generally, in reproducing apparatus such as an electronic copying machine, light is applied to an original mounted on a document glass plate from a light source and the reflected light is transmitted through an optical system including a mirror and so forth to the surface of a photosensitive member thereby to form an optical image on the surface of the photosensitive member to record an image.
There are two types of apparatus of the kind described, namely, apparatus in which the optical system is moved while the document glass plate is stationary and apparatus in which the document glass plate is moved while the optical system is stationary.
FIG. 1 schematically shows an exposure device of the type in which the optical system is moved. An original 1 is placed on a transparent document glass plate with the printed side thereof directed downwardly. As the original is illuminated by light coming from an illuminating lamp 3, the light reflected by the printed surface is transmitted to the surface of a photosensitive member 9 through reflecting mirrors 4, 5, 6 and 7. A reference numeral 8 denotes a focussing lens for forming an image. The optical system having the described construction operates in a manner explained hereinunder. As the illuminating lamp 3 lights up, the first reflecting mirror 4 movable integrally with the lamp 3 is moved at a velocity v in parallel with the original, as indicated by an arrow. The second reflecting mirror 5 and the third reflecting mirror 6 are moved in parallel with the original simultaneously with the movement of the first reflecting mirror 4. However, in order to maintain a light path of constant length, the second and third mirrors 5 and 6 move at a velocity of 1/2 v. In consequence, the entire surface of the original is scanned optically, so that the light reflected by the image is transmitted to the surface of the photosensitive member 9. Meanwhile, the surface of the photosensitive member 9 is rotated at the same velocity v as the first reflecting mirror 4, so that the reflected light is projected upon the surface of the photosensitive member 9 to form precisely a photo image of the original.
In the moving optical system described above, the second reflecting mirror 5 and the third reflecting mirror 6 are adapted to move as a unit with each other at a velocity which is a half of that of the first reflecting mirror 4. More specifically, the two reflecting mirrors are secured to a common reflecting mirror holder, and are moved along a rail which is usually disposed at the driving side for the holder by being towed by a wire of the like.
Reproducing apparatus such as an electronic copying machine having an optical system of the type mentioned above is becoming popular, thus giving rise to a demand for compact and light-weight construction which, in turn, requires a compact and light-weight exposure device.
On the other hand, the light source device consisting of illuminating lamp is adapted to move along a guide rail in unison with the holder for the first reflecting mirror. In conventional apparatus, electric power is supplied to the light source through a flexible power supply cord which stretches to follow the movement of the holder for the first reflecting mirror. In a moving type exposure device in which the light source device and the holder for the first reflecting mirror are moved along the guide rail, there is the problem that scanning is inconveniently unstabilized as the velocity of the moving exposure device is increased to meet the demand for higher copying speed. In addition, supplying power to the moving exposure device through a power supply cord to the light source device is not considered appropriate in view of current demand for compact construction of the apparatus.
As is well known, in an electronic copying machine, optical scanning of the original is made by synchronously moving a plurality of moving plates each carrying an exposure slit and a mirror. The synchronous driving of the plurality of moving plates is effected by means of a wire-pulley device stretched between one end of the moving plate and a stationary part of the apparatus. According to this single-side driving system, it is necessary to increase the holding span for the guide member, so that the size of the slit type exposure device is increased impractically. Therefore, in conventional apparatus, a so-called two-side driving system employing wire-pulley devices disposed at both sides of the moving plate has been adopted to decrease the holding span thereby to reduce the size of the apparatus as a whole. Even in such an improved apparatus, however, the plurality of moving plates are adapted to be moved along the same track surface. As a result, there is a practical limit by which the distance between the mirrors on the adjacent moving plates can be reduced, and the size of the apparatus as a whole cannot be reduced to such an extent as desired.
In the slit exposure optical system in which the first and second moving plates of the optical system are mounted on the guide member, the first and second moving plates must be of certain weights. Specifically, if the weight of a moving plate is reduced unlimitedly, the moving plate will float above the track surface undesirably and cause a disturbance in the image. However, from the viewpoints of improvement in the start-up characteristics of the slit exposure device and reduction in size of the driving motor, it is preferred that the weight of the first and second moving plates be reduced.
Conventionally, the mirror assembly has been produced by die-casting and the mirror mounting surfaces are finished highly precisely because it is necessary to mount the two mirrors with high optical precision. This arrangement, however, increases the weight of the mirror assembly inconveniently, although sufficient precision is obtained, so that the driving torque which has to be exerted by the wire-pulley device is increased. Moreover, die-casting and fine finishing uneconomically raises the cost of production. Therefore, recently, it was proposed to form a part of the mirror assembly by plate-work. This, however, makes it difficult to obtain the desired optical precision and requires fine adjustment using spacers or the like in mounting the mirrors. This is disadvantageous from the standpoint of labor and time.